Micromechanical components which are used in the automotive field, as inertial or acceleration sensors, for example, normally have a micropattern having movable functional elements. The micropattern is also designated as a MEMS structure (microelectromechanical system). During the operation of the sensors, the deflection of a functional element is detected, for instance, by a change in the electrical capacitance compared to a fixed reference electrode.
Different methods may be performed to produce micromechanical component. One possible procedure is to develop functional or MEMS patterns based on CMOS production techniques (complementary metal oxide semiconductor). Conventionally, for instance, in CMOS process technology, one may use produced metallic circuit traces as free-standing, movable functional elements and as evaluation electrodes. For the (partial) exposure of circuit traces, an oxide that is lying under the circuit traces and is used as a sacrificial layer is removed. A disadvantage is, however, that (exposed) metal circuit traces have relatively poor mechanical properties. Also, conventional CMOS processes have proven unsuitable for producing structures having a high aspect ratio.
There are therefore alternative approaches in which the circuit traces remain connected via oxide layers to an associated silicon substrate. Furthermore, the substrate is patterned in such a way that silicon crosspieces are produced below the circuit traces. In such a micropattern the mechanical properties are essentially determined by the silicon crosspieces, which surpass the properties of pure exposed metal circuit traces. Then, too, silicon structures having a high aspect ratio are able to be produced.
The thickness and the height of a circuit trace pattern (typically 5 to 10 μm) produced according to CMOS process technology and used as an electrode is (generally) smaller than the electrode height in components having functional structures made of pure silicon (for instance, 10 to 100 μm). In such a process it is therefore important to enable the production of small patterns at low distances. One challenge is particularly the forming of narrow, deep trenches in an oxide.
Anisotropic plasma etching processes that are usable for oxide etching, however, have a tendency to produce positively converging etching sides. The circuit traces which are possibly used as etching masks may also be exposed to metal removal. Anisotropic plasma etching processes therefore limit the minimally attainable distance between metal circuit traces and the maximum attainable etching depth. By contrast, isotropic oxide etching methods result in a lateral slight etching in the area of etching accesses between individual circuit traces. Therefore, in the case of functional structures, which are constructed from a plurality of circuit traces, situated one over the other and connected via oxide layers in an insulating manner, complete underetching of the circuit traces may occur. This can be avoided by laying out the circuit traces to have a large width. Since the etching rate between the individual oxides can be very different, depending on the production, but this approach is connected with relatively wide electrode structures.